Protein accumulation, modifications and aggregation are pathological aspects of numerous metabolic diseases including well known neurodegenerative diseases such as Huntington's, Alzheimer's (AD) and Parkinson's diseases (PD) (Taylor et al., Science 296 (2005), 1991-1995). Pathological protein aggregation is also involved in metabolic diseases such as diabetes mellitus type 2 (T2D) and islet rejection following clinical pancreatic islet transplantation into individuals with diabetes mellitus type 1 (T1D). Misfolding and aggregation of proteins lead to the development of amyloid deposits and seem to be directly related to cell toxicity in these diseases. Islet amyloid polypeptide (IAPP or amylin), a physiological peptide co-secreted with insulin by β-cells in the pancreas, forms fibrillar aggregates in pancreatic islets (also called islets of Langerhans) of T2D patients and has been suggested to play a role in the development of the disease (Westermark et al. (2011), Physiol. Rev. 91(3): 795-826). Furthermore, as mentioned before, IAPP aggregates have been found in pancreatic islets upon transplantation of isolated islets in patients with diabetes mellitus type 1 (T1D).
Human IAPP (hIAPP) is a peptide hormone that consists of 37 amino acids, with a disulfide bridge between cysteine residues 2 and 7 and an amidated C-terminus. Pancreatic islets are composed of 65 to 80% β-cells, which produce and secrete insulin and IAPP essential for regulation of blood glucose levels and cell metabolism. IAPP is processed from preprohormone preproIAPP, a 89 amino acid precursor produced in pancreatic β-cells.
PreproIAPP is rapidly cleaved after translation into proislet amyloid polypepide, a 67 amino acid peptide, which undergoes additional proteolysis and post-translational modifications to generate hIAPP. hIAPP expression is regulated together with insulin, as increased insulin production leads to increased hIAPP levels. hIAPP is released from pancreatic β-cells into the blood circulation and is involved in glycemic regulation through gastric emptying and satiety control, in synergy with insulin.
While hIAPP acts as a regulator of cell metabolism under physiological conditions, hIAPP can aggregate and form amyloid fibrils (IAPP amyloidosis) associated with β-cell failure, increased β-cell death and reduced β-cell mass. Several evidences point toward hIAPP amyloidosis as a major trigger for T2D pathogenesis. First, deposition of hIAPP fibrils is found in more than 90% of type-2 diabetes patients (Zraika et al. (2010), Diabetologia 53(6): 1046-1056). Second, hIAPP aggregation is toxic to β-cells and correlates with the reduction in insulin producing β-cells (Butler et al. (2003), Diabetes 52(9): 2304-2314; Ritzel et al. (2007), Diabetes 56(1): 65-71; Jurgens et al. (2011), Am. J. Pathol. 178(6): 2632-2640). Third, transgenic murine models expressing hIAPP show pancreatic islet amyloid deposits and spontaneously develop T2D (Janson et al. (1996), Proc. Natl. Acad. Sci. USA 93(14): 7283-7288; Hoppener et al. (1999), Diabetologia 42(4): 427-434; Hull et al. (2003), Diabetes 52(2): 372-379; Butler et al. (2004), Diabetes 53(6): 1509-1516; Matveyenko et al. (2006), ILAR J. 47(3): 225-233; Hoppener et al. (2008), Exp. Diabetes Res. 697035). They recapitulate the human disease with β-cell dysfunction, β-cell mass deficiency and β-cell loss, comparable to what observed in the tissues from T2D patients. hIAPP expression and amyloid formation directly correlate with β-cell apoptosis and diabetes development in these models, thus providing evidence for the contribution of human IAPP in the development of the disease. Moreover, treatment interfering with hIAPP aggregation ameliorated the diabetic phenotype and increased animal life span (Aitken et al. (2009), Diabetes 59(1): 161-171). hIAPP aggregation and amyloidosis is a prerequisite for toxicity. The non-amyloidogenic rodent IAPP (rIAPP), which is unable to form fibrils as a result of six amino acid substitution, is nontoxic to β-cells. In the development of the disease, pathological hIAPP aggregation found in human pancreatic islets may cause β-cell dysfunction and death associated with impairment of insulin secretion. In addition, compensatory increase in β-cell mass and insulin and amylin secretion to maintain normal blood glucose levels may favor the formation of toxic hIAPP oligomers and deposition of hIAPP fibrils. While initial hIAPP oligomers are considered as the main cytotoxic species, the hIAPP fibril end product may also play a role in β-cell loss (Meier et al. (2006), Am. J. Physiol. Endocrinol. Metab. 291(6): E1317-1324; Haataja et al. (2008), Endocr. Rev. 29(3): 303-316; Engel et al. (2008), Proc. Natl. Acad. Sci. USA 105(16): 6033-6038). hIAPP fibrils have also been observed in isolated pancreatic islets from donors and associated to β-cell loss following clinical pancreatic islets transplantation into individuals with type-1 diabetes (Andersson et al. (2008), Exp. Diabetes Res. 562985; Udayasankar et al. (2009), Diabetologia 52(1): 145-153; Bohman et al. (2012), Amyloid 19(2): 87-93). The exact mechanism leading to hIAPP aggregation and amyloidosis in T2D is unknown. Insulin resistance in T2D increases insulin secretion demand together with proIAPP cell content and hIAPP release, what may elicit amyloidosis as hIAPP fibril formation is concentration dependent. Another proposed mechanism is the accumulation and aggregation of N-terminal unprocessed proIAPP caused by proteolysis failure in the setting of insulin resistance, as partially processed forms of proIAPP are found in amyloid deposits, in particular the 48 residue intermediate proIAPP1-48 (Marzban et al. (2006), Diabetes 55(8): 2192-2201). In this context, abnormal processing of proIAPP may act as a seed for hIAPP amyloidosis and increase amyloid formation (Paulsson et al. (2005), Diabetes 54(7): 2117-2125; Paulsson et al. (2006), Diabetologia 49(6): 1237-1246; Marzban et al. (2006), Diabetes 55(8): 2192-2201). ProIAPP is therefore also considered as an appropriate therapeutic target.
Clinical features of T2D are high blood glucose levels and insulin resistance and/or deficiency. Diabetes mellitus is a group of metabolic diseases including T1D, T2D, and gestational diabetes. T2D, also named adult-onset diabetes, obesity-related diabetes, and noninsulin-dependent diabetes mellitus (NIDDM) is the most common form of diabetes, accounting for about 90% of all cases (Gerich et al. (1998), Endocr. Rev. 19(4): 491-503). T2D is characterized by a decrease in the number of functional insulin-producing β-cells. While the pathology progresses, it can lead to long-term complications such as cardiovascular disease, diabetic retinopathy leading to blindness, kidney failure, frequent infections, and amputations caused by poor circulation. As a consequence, T2D is associated with a shorter life expectancy. The disease affects more than 300 million people worldwide resulting in more than a million deaths annually. Both genetic determinants and environmental factors lead to the development of the disease, with obesity, physical inactivity and aging thought to be the primary cause (Kahn et al. (2006), Nature 444(7121): 840-846).
Current treatments for T2D include lifestyle management (diet and exercise) and pharmacological intervention such as metformin and insulin supply to decrease blood glucose levels by either stimulating the pancreas to release insulin or increasing insulin response. These treatments are based on symptomatic improvement of diabetes, with the consequence of a lack of durability. Indeed, none of the available treatments have been shown to counteract the aggregation of hIAPP and the loss of pancreatic β-cells. New treatment strategies involving analogues of glucagon-peptide 1 (GLP-1) (Butler et al. (2009), Diabetologia 53(1): 1-6) and inhibitors of GLP-1 inactivating enzyme dipeptidyl-peptidase 4 (DDP4) are based on the potent insulinotropic effect of GLP-1 and its effect to enhance β-cell proliferation. Importantly, increased insulin release is also coupled to increased amylin release. Experimentally, stimulated insulin secretion has been shown to promote the development of islet amyloidosis in animal models and similar effects can be expected in humans (Aston-Mourney et al. (2011), Diabetologia 54(7): 1756-1765). These treatments could therefore potentially aggravate islet amyloidosis. More recent and promising strategies involve the development of anti-inflammatory drugs or antibodies targeting the IL-1β pathway (Donath et al. (2008), Nat. Clin. Pract. Endocrinol. Metab. 4(5): 240-241; Ehes et al. (2009), Proc. Natl. Acad. Sci. USA 106(33): 13998-14003; Owyang et al. (2010), Endocrinology 151(6): 2515-2527; Dinarello et al. (2010), Curr. Opin. Endocrinol. Diabetes Obes. 17(4): 314-321; Boni-Schnetzler et al. (2011), J. Clin. Endocrinol. Metab. 93(10): 4065-4074; Boni-Schnetzler et al. (2012), Br. J. Clin. Pharmacol.; Cavelti-Weder et al. (2012), Diabetes Care). Of important note, recent studies show that hIAPP specifically induce the inflammasome—IL-1β system leading to activation of the innate immune system (Masters et al. (2010), Nat. Immunol. 11(10): 897-904; Mandrup-Poulsen et al. (2010), Nat. Immunol. 11(10): 881-883), thus supporting a therapeutic strategy targeting hIAPP aggregation.
These findings highlight the potential benefit associated with active or passive immunotherapy approaches targeting hIAPP and/or proIAPP.
Summarizing the above, novel therapeutic strategies are urgently needed addressing aggregated hIAPP, proIAPP proteins and/or hIAPP oligomers and/or fibrils with efficacious and safe therapy.
Passive immunization with human antibodies which are evolutionarily optimized and affinity matured by the human immune system would provide a promising new therapeutic avenue with a high probability for excellent efficacy and safety.